Integrate SqlAlchemy dialect compliance suite into testing framework

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We recently encountered an issue with the CrateDB sqla dialect (#348). SqlAlchemy provides a dialect compliance testing suite (https://github.com/sqlalchemy/sqlalchemy/blob/master/README.dialects.rst) to ensure dialects adhere to certain compliance standards - which would have identified this issue before it was reported by an external user.

However, in the past, it seems we had problems with the compliance suite due to a lack of transactions. We should:

1. Investigate if this is still the case with the testing suite
2. Integrate the compliance suite into our testing